Arrange the following cations in decreasing order of stability: (I) 4-methoxybenzyl cation, (II) benzyl cation, (III) 4-nitrobenzyl cation.

In 4-methoxybenzyl cation, an oxygen lone pair is donated to the benzylic carbon through the ring (+M), giving an extra oxonium-type contributor. Benzyl is stabilised only by the ring. In 4-nitrobenzyl cation, the -M and -I effects of nitro place or intensify positive charge near the carbocation, which destabilises it. The order is I > II > III.